The present invention refers to a parison and a related manufacturing process adapted to the production, on an industrial scale, of containers of thermoplastic resin, particularly polyethylene terephtalate (PET) and polypropylene (PP) intended for filling with liquids that may also be at a high temperature and/or carbonated, i.e. containing CO.sub.2 gas (carbon dioxide).
In the field embracing the technology and the machines for manufacturing such containers there are a number of developments and improvements aimed at obtaining production processes and related apparatuses that are capable of producing the containers in an increasingly reliable, cost-effective, versatile manner, to an increasing level of quality, in a highly competitive industrial context of very large-scale production. These production processes are generally known to be capable of being schematically grouped into two basic typologies, i.e. single-stage and two-stage processes. The present invention applies to parisons obtained with both such types of processes, as well as the same respective processes.
Single-stage processes are so defined in that they are capable of forming the so-called preform, or parison, and transferring the parison from the injection mould or extrusion die (upon it having been cooled down to some appropriate temperature) to a conditioning station, where it is allowed to uniformly level at a temperature of preferred molecular orientation. The preform or parison is then transferred to a blow-moulding mould in which it is finally moulded into its desired form.
Inherent to any single-stage process is the fact that an uneven heat distribution takes place across the cross-section area of the wall thickness of the parison when the latter is transferred from the injection mould or extrusion die. Various processes have been patented concerning the times and the temperatures of the parison when it is removed from the injection mould with a view to optimizing the cycle times.
The patent literature covering single-stage processes discloses in all cases a final forming or moulding of the thermoplastic resin container which is in some way allowed, through a conditioning station, to gain a uniform wall temperature throughout the cross-section area thereof, such a temperature corresponding to the preferred molecular orientation temperature of the resin.
Two-stage processes are so defined due to the fact that the blow-moulded bottle is obtained in two distinct phases which may be carried out even at guite great intervals between each other. In fact, the actual advantage of this technology derives exactly from the circumstance that the whole process is divided into two phases that are normally carried out widely apart from each other in terms of both place and time, thereby ensuring greater flexibility from a technical, manufacturing and marketing point of view.
The individual parisons are produced in the first phase of the above process, wherein the parisons are then usually stored in situ or transported to the premises of the final user or processor.
In the second phase of the above process, the parisons are then re-conditioned to the desired temperature and, immediately thereafter, blow-moulded into the desired final products, ie. the bottles.
In addition to such a greater flexibility, two-stage processes also potentially enable considerable economies of scale to be obtained, since a single manufacturer is able to produce, in a single and same plant, the parisons that can then be used to produce various different types of bottles.
However, two-stage processes have a major inherent drawback in their greater energy usage due to the fact that, in the second phase or stage thereof, the parisons must be fully re-conditioned, ie. heated to the optimum temperature required for the subsequent blow moulding operation.
In both the single-stage and two-stage processes and plants for the production of hollow plastic products, typically bottles, parisons are obtained through continuous extrusion of a flow of thermoplastic resin, in particular polyethylene terephtalate (PET), into a plurality of multiple moulds. However, the actual production of a parison is not independent from the manner in which it is going to be blow moulded, as well as the manner in which the resulting bottle is going to be used, but must on the contrary take due account of all such variables, mainly:
the shape of the bottle, PA1 the internal volume of the bottle, PA1 the type of liquid which is going to be filled in the bottle and which may be either highly or normally carbonated or even simply "plain", i.e. uncarbonated, PA1 the state of the liquid, which can be either hot or cold when filled, and PA1 the way in which the bottle itself is going to be used, since it can be designed for disposal after use or be of the re-usable type and, as a result, refillable for a number of times after recovery, cleaning and sanitation, etc. PA1 three hours at 60.degree. C. with 25% of NaOH, followed by twenty-four hours at four volumes of CO.sub.2 at 38.degree. C. PA1 1) it shall avoid everting, ie. bending outwards; PA1 2) it shall avoid giving rise to stress-cracking or any other breakage of that kind; PA1 3) it shall not cause the bottle to become unsteady on its resting base or plane; PA1 4) it shall maintain full perpendicularity of the axis of the bottle relative to the resting base or plane. PA1 1) Greatest possible orientation of the amorphous zone 1. This is obviously the zone in which the orientation tends to be reduced to a minimum, while material concentration is the greatest. Since there is practically no material orientation, mechanical strength is ensured solely by the thickness thereof. However, an excessively great thickness would only mean a waste of material. Up to 25% of the overall weight of refillable bottles is in the bottom portion thereof (zone 1+zone 2). It is therefore necessary for this amorphous zone 1 to be given the greatest possible orientation, although this may prove extremely difficult to be obtained. PA1 2) Greatest possible orientation of the amorphous zone 2. These are the most critical zones, since material orientation and distribution are decisive for the mechanical strength of the bottom. PA1 a) criticalness of process tolerances, known also as "process window", when blow moulding parisons with a substantially constant thickness of their walls, due to the fact that the material is at the same starting temperature, but must be stretched to very different values, while keeping pre-defined minimum thicknesses in the most critical zone of the resting base of the bottle; PA1 b) the use of parisons with a constant wall thickness has the consequence that the final bottle happens to have some of its zones with a significantly greater thickness than the one which would be actually required to withstand the respective stresses or loads, so that it can be concluded that at least a part of the material used in the zones is practically wasted; PA1 c) in single-stage plants, introducing the temperature conditioning phase before the actual blow moulding phase gives rise to a number of problems, the solution of which implies high costs and greater complications in the construction of the related plant, as well as in the process performed in the same plant. PA1 a) a parison for use in a single-stage process for producing a container of thermoplastic resin which is thermally stable, capable of being filled with both hot liquids and carbonated liquids, capable of being re-used for a number of times, and provided with a `champagne`-type bottom offering the afore mentioned performance capabilities, and PA1 b) a parison for use in a two-stage process for producing containers of a type similar to the one described above with reference to single-stage processes, which further enables material usage to be reduced to a minimum,
With reference to the shape of re-usable, or "refillable" bottles, as they are generally referred to in the art, it has been observed that these prove to be particularly well-suited to re-utilization if their bottom, or base, is given a "champagne" bottle-like shape, instead of a "petaloid"-like one, these two terms being generally and unmistakably known to those skilled in the art.
This is practically due to a twofold reason, ie. to the petaloid bottom being much more subject to crackings and breakages (stress cracking problems) during the subsequent treatments of the bottles, and to the cleanability of the bottle, in view of the re-utilization of the same, being clearly much poorer in the deep recesses of the petaloid, owing to it being hindered by the particular receptacle-like shape of said recesses.
The main problem encountered in the manufacturing of refillable bottles lies in trying to ensure an optimum distribution of the material, particularly in the area of the resting base of the bottle, and therefore a differentiated thickness pattern along the walls of the bottle and, as a result, a differentiated thickness pattern even on the parison. In fact, in view of its adequate mechanical strength at the various pressure and temperature conditions, the bottom should possess a well-defined material thickness map.
It is generally known that, while the internal pressure strength of a bottle with a petaloid bottom is given by the geometry, ie. the actual shape of the same bottom, and in particular by the ribs (tie-beams) thereof, only an adequate and properly distributed quantity of material will be able to ensure the same function of the petaloid configuration in a `champagne` type of bottle bottom.
For instance, in the case of 2-liter bottles the `champagne`-type bottom must have performance abilities to withstand the following treatment:
When submitted to the above cited test conditions, the bottom of a refillable-type bottle must have the following performance:
For such performance capabilities to be reached, a `champagne`-type bottle bottom must be given certain characteristics, which, with reference to FIG. 1, are as follows:
All `champagne`-type bottoms submitted to inner pressure must have minimum thickness values at the points generally indicated at A and A1, below which the bottom would unfailingly give way. In addition to this imperative requirement, the bottoms must also comply with another basic requirement, ie. they must have an adequately uniform thickness pattern along their circumference in correspondence of the zones 2. Only a minimum thickness difference can be at best allowed between two diametrically opposite points.
For instance, in the case of a `champagne`-type bottom of a 2-liter refillable bottle, the greatest allowable thickness difference amounts to 0.2 to 0.25 mm. It is important for this difference to be kept as small as possible in view of minimizing non-perpendicularity of the bottle.
The extent of orientation is certainly very important, but not as important as the manner in which the material distributes itself in these zones. In both above mentioned methods, such a distribution is furthermore strongly dependent on the temperature conditioning treatment which the parisons are caused to undergo before being blow moulded, and such a conditioning treatment implies a twofold drawback: on the one side, the parison wall is heated up in a substantially uniform manner and this leads to a fairly uniform stretching of the material during blow moulding, ie. gives rise to a conflicting situation with the requirement of a differentiated material distribution, according to a well-defined pattern, along the various zones of the bottle; on the other side, such a conditioning phase of the process, in single-stage processes, slows down and complicates the entire production process due to a variety of technical and economical reasons, as those skilled in the art are well aware of.
As far as two-stage processes and related parisons are concerned, it is a fact inherent thereto that, being it necessary, due to inherent plant limitations, for the parison to be conditioned to a substantially uniform temperature, while it is on the other hand necessary for the bottom of the same parison to then be stretched according to a very differentiated pattern, it ensues that, in order to ensure a satisfactory strength of the base, or contact zone, which usually undergoes greater stretching, the need arises for the use of parisons having an adequate thickness. However, the thickness required for the highly stretched zones turns out to be actually excessive for the other zones of both the bottom and the remaining body portions of the parison, so that such a thickness constraint conclusively translates into an ineffectual consumption of plastic material used in those portions of the bottles that undergo a low-to-medium level of stretching.
In order to reduce such an ineffectual utilization of material, efforts are made in view of keeping the conditioning temperature within very tight limits; however, controlling such a process parameter is not always easily or economically possible.
From U.S. Pat. No. 5,158,817 is known the shape of a preform for a generally rectangular container, specifically of an oval or oblong cross section. However such kind of preform is not best used for bottles having a cylindrical shape and a bottom of the "champagne" type.
From WO 90/04543 a particular shape of preform is known. However such document does refer to bottles for one-piece disposable use (i.e. single service, see row 2 of the relevant description); therefore such kinds of preforms are not optimized for "refillable or returnable type" bottles, which is the object of the present patent application.
From EP 0 445 465, the construction of preform for returnable bottles is known; however the illustrated preforms are used for generic purpose and processing, while it is well known that, due to different heating requirements in the one-stage and bi-stage process, the relevant preforms have to be shaped accordingly and differently.
The basic drawbacks of the present technology for the production of parisons can therefore be summarized as follows: